"Twitter cards make it possible for you to attach media experiences to Tweets that link to your content. Simply add a few lines of HTML to your webpages, and users who Tweet links to your content will have a "card" added to the Tweet that’s visible to all of their followers." - dev.twitter.com
For a little while we've been able to embed tweets in pages that bring with it the ability for for readers to follow people, reply and re-tweet directly from that page. Now, when we include a link in a tweet, it means we can also specify what information from that linked page is shown alongside that tweet in the form of a nice little Twitter card. These come in 3 varieties - Summary (see below), Photos (more around a specific image, eg an Instagram photo or a Kickstarter video) and Players (more about the person themselves).
Whilst the new Twitter cards feature is still in somethin of a beta state (you have to fill in a short form and wait for Twitter to verify you've done it correctly) and don't yet extend to a tweet embedded in a page, it is still a nice feature that allows content producers to create a richer experience for their readers and drive more traffic to their site.
If you need us to work on your blog or website to get Twitter cards working on there, get in touch.